In general, a prior-art personal digital assistant (PDA) usually inputs data by a touch-screen pen, and also inputs messages by a multiple of keys the same way as a mobile phone does. However, the external keyboard becomes a very important accessory for inputting data for long time. Therefore, the way of integrating the aforementioned products and the keyboard to maximize the utility of the keyboard and minimize its size to facility the carrying when the keyboard is not in use is definitely an important subject for the research and development of the present high-tech external keyboard structure.
Furthermore, to provide a sufficient number and appropriate size of the keys, the structure of an ordinary external keyboard must be large enough and sometimes is even larger than a handheld device, such as the personal digital assistant itself, making the carrying very inconvenient. Moreover, considering the overall layout and minimization of size, many common keys are omitted or compound keys are used as substitutes, which makes the operation very inconvenient as well. Further, an ordinary external keyboard can be connected and fixed by a connector without additional support. If there is a slight vibration or shaking, it is common to see that the keyboard is detached or the signal is interrupted. Therefore, such kind of keyboard is limited for the use on a stable surface only, and cannot be used for a handheld operation, not mentioned the use in outdoors or even in a motor vehicle. It definitely affects the mobility of the handheld device.